Apparatus and method for igniting plasma in a process module

ABSTRACT

The invention provides apparatus and methods for improving systems that expose samples to reactive plasmas, and more particularly for igniting plasma within a process module. The systems are of the type which have an electrode pair and a radiofrequency generator connected to one electrode. Gas is injected between the electrodes where it is ionized and transformed into a plasma. The invention includes (i) ignition means for ionizing gas, e.g., silane, between electrodes which are separated by a small gap of less than approximately one centimeter; and (ii) a radiofrequency energy generator that preferably operates at high frequencies, e.g., 60 MHz, to transform molecules into plasma. Several embodiments of ignition means are taught by the invention, including: an electron source, an ultraviolet source, a second radiofrequency energy generator, and radioactive sources, among others. A process module constructed according to the invention, using high frequency energy and small electrode separations, has a high rate of deposition and a high production yield.

This application is a divisional application of Ser. No. 08/183,529filed on Jan. 19, 1994 now U.S. Pat. No. 5,565,036. The contents of allof the aforementioned application(s) are hereby incorporated byreference.

BACKGROUND

This invention relates to apparatus and methods for exposing samples toreactive plasmas. More particularly, the invention relates to apparatusand methods for igniting plasma within a process module that is, forexample, suitable for use in Chemical Vapor Deposition (CVD) and inPlasma Enhanced Chemical Vapor Deposition (PECVD) processes.

As used herein, a "sample" generically describes a substrate, such as aglass panel or a silicon wafer, which is suitable for depositiontechniques, for example by PECVD. The sample typically has one surfacethat is treated to receive one or more film depositions.

Vacuum deposition systems which deposit semiconducting or insulatingfilms onto samples are well-known and are utilized in a wide range ofscientific fields. Complex PECVD systems have been developed, forexample, to manufacture devices such as thin film transistors (TFTs),liquid crystal displays (LCDs), flat panel displays (FPDs), solar cells,photodetectors, and integrated circuit structures.

Typically, these systems include one or more evacuated process modulesthat are used to expose the sample to reactive plasmas. Such a processmodule typically includes first and second electrodes within a chamberand spaced apart to form a gap therebetween: one electrode iselectrically grounded to the chamber, and the other electrode isconnected to a radiofrequency (RF) source which generates aradiofrequency discharge within the gap. The module also has gas inletsand gas outlets which direct selected gases, such as silane, into andthrough the gap.

To deposit a selected film on a sample, the sample is suspended in thegap spaced away from the active RF electrode, e.g., mounted to thegrounded electrode, and a selected gas introduced into theinter-electrode gap. When the gas is exposed to the RF field, it ionizesand forms a reactive plasma which deposits a film onto surfaces exposedto the plasma, including the sample surface. The rate at which this filmis deposited onto the sample surface is dependent upon several factors,including: the magnitude of the vacuum in the process module; theelectrode spacing; the power and the frequency of the RF energy; and thegas flow rate.

For practical reasons, many prior vacuum deposition systems utilizeexcitation frequencies of approximately 13.56 MHz. However, it is knownto be beneficial to deposit films at higher frequencies, since fasterdeposition rates are realized. U.S. Pat. No. 4,933,203, for example,discloses the deposition of hydrogenated amorphous silicon atfrequencies from 30 MHz to 150 MHz, realizing a 500% to 1000% increasein deposition rates over those attained with 13.56 MHz systems. Suchincreases correlate to improved manufacturing throughput andefficiencies.

Nevertheless, further increases in deposition rates are sought to attainfurther improvements in manufacturing throughput and efficiency. Oneknown technique for increasing deposition rates is to decrease thespacing between the two chamber electrodes. However, the reduction ofthe spacing is not without limit. U.S. Pat. No. 4,933,203 discloses, forexample, that the spacing has a practical lower limit of ten millimetersbecause of certain phenomena. These phenomena include difficultiesassociated with igniting plasma at small electrode spacings.

Accordingly, it is one object of this invention to provide apparatus andmethods for depositing films onto samples from a vapor.

It is another object of the invention to provide improved CVD and PECVDapparatus and methods for igniting a plasma within process modules thathave small electrode spacings.

More particularly, an object of the invention is to provide PECVDapparatus and methods which increase the deposition rate of filmsdeposited on samples exposed to reactive plasmas.

Another object of the invention is to provide vacuum deposition systemsthat expose samples to reactive plasmas, and which attain relativelyhigh production yield.

Further object of the invention is to provide such apparatus and methodsfor exposing samples to reactive plasma and which are relatively low incost and reliable in operation.

Yet another object of the invention is to provide apparatus and methodsof the above character and which also can etch sample surfaces.

These and other objects of the invention will be apparent in thedescription which follows.

SUMMARY OF THE INVENTION

The invention attains the aforementioned objectives by providing, in oneaspect, improvements to plasma process modules which expose samples toreactive plasma. These modules include first and second radiofrequencyelectrodes and a radiofrequency generator connected to the secondelectrode. In accordance with the invention, the first and secondelectrodes are spaced apart to form a substantially uniform gaptherebetween of less than approximately one centimeter.

The improvement also features an ionizing element for ionizing gasinjected between the electrodes. The ionizing element ignites the plasmawithin the electrode gap and the radiofrequency energy generatorafterwards transforms molecules within the module into plasma bygenerating RF energy at a primary frequency that is preferably sixtymegahertz.

The improvement preferably comprises a gas that is injected between theelectrodes. The gas is of a type that is suitable for ionization andthat is suitable for transformation into a plasma when exposed toradiofrequency energy at the primary frequency. In accord with theinvention, the gas is generally selected from gases such as silane(SiH₄), disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine(PH₃), nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon(Ar), carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆),chlorine (Cl₂), sulfur hexafluoride (SF₆), hydrogen chloride (HCl),carbon tetrachloride (CCl₄), hydrogen bromide (HBr), carbon dichloridedifluoride (CCl₂ F₂), boron trichloride (BCl₃), silicon tetrachloride(SiCl₄), boron tribromide (BBr₃), chlorotrifluoride (ClF₃), fluorine(F₂), and mixtures thereof.

To inject a gas between the electrodes, the improvements thus include,in other aspects, gas inlets and gas outlets, and associated pumpmechanisms. The pump mechanisms pump gas through the gas inlet and intothe gap, and extract gas and other molecules through the gas outlet asprocess waste.

In another aspect, the invention provides an electron source with afilament as the ionizing element. The electron source is arranged toinject electrons between the electrodes at an energy that is preferablyat least 300 eV to ionize gas in the gap. In certain aspects, theelectron source is spaced selectively at a distance from the gap toselect the electron energy entering the gap. Alternatively, the sourceincludes an extraction slit that has a selectively variable positivepotential relative to the filament, for extracting electrons from thefilament with energies of variable magnitude and greater than 300 eV.

In still another aspect, the improvement provides an ultraviolet lightsource as the ionizing element and which irradiates ultravioletradiation between the electrodes to ionize gas in the gap. Theultraviolet light source preferably generates photons with energies thatare greater than approximately five electron volts. In certain preferredaspects, the improvement includes an UV interface that is transmissiveto the ultraviolet radiation generated by the UV source, so that the UVsource generates photons which irradiate the gap through the UVinterface. Such an ultraviolet light source is preferably located instandard atmosphere outside the module, which is evacuated, and isarranged to illuminate the vacuum of the evacuated module through the UVinterface.

In certain aspects, the ultraviolet light source includes (i) areflector and an aperture which, in combination, collect the emittedultraviolet radiation and transmit the radiation in a concentrateddirection toward the gap in the module; and (ii) one or more opticalelements which collect and collimate the transmitted concentratedradiation from the reflector to the within the gap.

In still other aspects according to the invention, the improvementprovides a sparking apparatus as the ionizing element. The sparkingapparatus generates a spark or other electric discharge between theelectrodes to ionize gas in the gap. Preferably, therefore, the sparkingapparatus is isolated from the ground of the process chamber and has anexposed spark gap arranged with a line of sight from the spark gap tobetween the electrodes, such that an emitted spark at the spark gapinjects electrons to within the gap.

In yet another aspect of the invention, the improvement provides asecond radiofrequency generator that is connected to the secondelectrode. The second radiofrequency generator has a second frequency,preferably 400 kHz, that is less than the primary frequency and thatgenerates radiofrequency discharges between the electrodes at the secondfrequency to ionize gas in the gap. In certain preferred aspects, thesecond radiofrequency generator pulses the second frequency at aselected rate. It is also preferred, according to the invention, toinhibit the second radiofrequency generator once gas is ionized withinthe gap. The two radiofrequency generators typically operate incombination to generate a voltage between approximately 1 kV and 5 kVacross the two electrodes.

A process module according to other aspects of the invention includes ahigh direct current power source that is connected to the secondelectrode. Such a direct current power source generates a physical sparkbetween the electrodes and accordingly ionizes gas in the gap.

In another aspect, a process module of the invention includes evacuationapparatus, e.g., a pump and/or compressor, that evacuates the moduleselectively to approximately 0.01 Torr. Preferably, such a processmodule also includes pressurizing apparatus, e.g., a pump and/orcompressor, which pressurizes the module selectively. In accordance withthe invention, the improvement provides for alternately controlling thepressure within the module to selectively pressurize the module for ashort duration within which ionization of gas occurs. For example, themodule is first evacuated by a pump to approximately 0.01 Torr so thatoxidation and sample impurity is kept to a minimum. Thereafter, adeposition or etch gas is injected between the electrodes and the pumppressurizes the module to approximately 0.5 Torr so that the gas isignited by the primary frequency generator. Once gas is ionized, themodule is depressurized to approximately 0.1 Torr which is a preferreddeposition or etch operational pressure.

In still another aspect, the process module of the invention includes anx-ray source, or alternatively a radioactive source, as the ionizationelement. The x-ray or radioactive source is arranged to generate a beamof x-ray or radioactive radiation into the gap to ionize gas therein.Preferably, to enhance user safety, such a source is shielded to confinethe x-ray or radioactive radiation substantially to one or more pathsextending from the source and through the gap.

The improvements of the invention also include a plurality of gaseswhich operate in combination with one another as the ionization element.For example, a first primary gas can include silane (SiH₄), disilane(Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃), nitrogentrifluoride (NF₃), helium (He), carbon tetrafluoride (CF₄),hexafluoroethane (C₂ F₆), oxygen (O₂), nitrous oxide (N₂ O), methane(CH₄), borane (BH₃), diborane (B₂ H₆), chlorine (Cl₂), sulfurhexafluoride (SF₆), hydrogen chloride (HCl), carbon tetrachloride(CCl₄), hydrogen bromide (HBr), carbon dichloride difluoride (CCl₂ F₂),boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), borontribromide (BBr₃), chlorotrifluoride (ClF₃), fluorine (F₂), and mixturesthereof. A second gas, such as helium (He), argon (Ar), krypton (Kr),nitrogen (N₂), xenon (Xe) neon (Ne), and mixtures thereof, is combinedwith the first gas to form a gaseous mixture that is transformable intoplasma by the primary frequency. Preferably, therefore, the mixture ofthe two gases is ignitable by a radiofrequency source that generates 60MHz discharges. Once the mixture of the two gases is ignited, the secondgas is inhibited from flowing into the module, so that deposition onto asample surface results without influence from the second gas.

In still another aspect, the improvement of the invention includes amotion actuator, e.g., a linear actuator, as the ionization element andwhich moves at least one of the electrodes selectively to change thedimension of the gap between the electrodes. The motion actuatorseparates the electrodes to a first dimension, at which theradiofrequency generator, of high primary frequency, ionizes gasinjected between the electrodes. Preferably, the motion actuatorthereafter reduces the gap spacing to below approximately onecentimeter, a desired dimension for high rate plasma deposition.

In yet another aspect, the improvement includes an atomic element as theionization element and which has an ionization energy of no more thantwenty-five electron volts. The atomic element, such as helium or argon,is injected into the gap to ionize gas therein.

The invention also provides a sample-processing method for exposing asample to reactive plasma within a process module having first andsecond electrodes. In particular, the improvement includes the steps of(i) arranging the first and second electrodes within the module to forma substantially uniform gap separating the electrodes of betweenapproximately one and ten millimeters; (ii) injecting a gas, such assilane (SiH₄), disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃),phosphine (PH₃), nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He),argon (Ar), carbon tetrafluoride (CF₄), carbon tetrachloride (CCl₄),hexafluoroethane (C₂ F₆), oxygen (O₂), nitrous oxide (N₂ O), methane(CH₄), borane (BH₃), diborane (B₂ H₆), and mixtures thereof, into thegap; (iii) ionizing the gas; and (iv) applying radiofrequency energy tothe gap which has a primary frequency, e.g., 60 MHz, that transformsmolecules into plasma.

In one aspect, the method includes the step of injecting a second gasinto the gap wherein the first and second gases form a gaseous mixturethat is transformable into plasma by the primary frequency. Preferably,the second gas is helium (He), argon (Ar), krypton (Kr), nitrogen (N₂),xenon (Xe) neon (Ne), or mixtures thereof.

In still another aspect, the method according the invention includes theadditional steps of (i) injecting into the module an etch gas, such aschlorine (Cl₂), sulfur hexafluoride (SF₆), hydrogen chloride (HCl),carbon tetrachloride (CCl₄), hydrogen bromide (HBr), carbontetrafluoride (CF₄), hexafluoroethane (C₂ F₆), nitrogen trifluoride(NF₃), carbon dichloride difluoride (CCl₂ F₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), oxygen (O₂), boron tribromide (BBr₃),chlorotrifluoride (ClF₃), fluorine (F₂), or mixtures thereof; (ii)ionizing the etch gas; and (iii) applying to the gap radiofrequencyenergy with a primary frequency, e.g., 60 MHz, to transform the etch gasinto an etch plasma such that surfaces in contact with the etch plasmaare etched. This method preferably includes the step of arranging theelectrodes to form a substantially uniform gap separating the electrodesof between approximately one and seventy five millimeters during theetching of the surfaces.

Other particularities associated with the deposition of certainsemiconductor coatings, e.g., amorphous silicon, are described in U.S.Pat. No. 4,933,203, which is herein incorporated by reference

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a process module constructed inaccordance with the invention;

FIG. 2 is a side view of the process module of FIG. 1 with a pressurepump and linear actuators operable to enable ionization of gas withinthe module;

FIG. 3 graphically shows the relationship between ionization andelectrode potential versus gap spacing and pressure for selected typicalgases;

FIG. 4 is a side view of the process module of FIG. 1 with an electronsource operable to enable ionization of gas within the module;

FIG. 5 is a side view of the process module of FIG. 1 with anultraviolet source, inside the module, operable to enable ionization ofgas within the gap;

FIG. 5A is a side view of the process module of FIG. 1 with anultraviolet source, outside the module, operable to enable ionization ofgas within the gap;

FIG. 6 is a side view of the process module of FIG. 1 with a sparkingsource operable to enable ionization of gas within the module;

FIG. 7 is a side view of the process module of FIG. 1 with a second RFgenerator connected to the active electrode and operable to enableionization of gas within the module; and

FIG. 8 is a side view of the process module of FIG. 1 with a x-raysource embedded within the active electrode and operable to enableionization of gas within the module.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS

FIG. 1 schematically illustrates a process module 10a constructed inaccordance with the invention. The module 10a includes a firstradiofrequency (RF) electrode 12, a second RF electrode 14, and a RFgenerator 16 interconnected with the second electrode 14. The electrodesare spaced apart at a distance "d" to form a substantially uniform gap18 between the electrodes 12 and 14. The distance "d" is less thanapproximately ten millimeters and preferably more than one millimeter. Apressure-tight housing 20 surrounds the electrodes 12 and 14 so that themodule 10a may be evacuated selectively, to between approximately 0.01and ten Torr, by methods known to those skilled in the art.

The module 10a includes gas inlet 24 and gas outlet 26: the inlet 24 isused to inject gas into the module 10a; and the outlet 26 is used toextract gas, particulates, and other molecules from the module 10a. Inoperation, a gas that is suitable for ionization and for transformationinto plasma, e.g., silane, is introduced through the inlet 24 and intothe gap 18. The gas is ionized, in accordance with further features ofthe invention described below, and transformed into plasma by RF energy,which preferably has a primary frequency of 60 MHz. The RF energy isgenerated by the generator 16 and transferred to the gap 18 via the RFconnection 28 and electrodes 12 and 14, thereby transforming moleculesof the injected gas into a plasma 30.

A film is deposited onto surfaces in contact with the plasma 30.Accordingly, a sample 32, e.g., a flat glass or silicon wafer withopposed surfaces, is introduced into the gap 18 in the module 10a toexpose the sample 32 to the plasma 30 for deposition of one or morefilms onto its surface. The sample 32 is typically mounted to the firstelectrode 12 such that the sample 32 is spaced away from the active RFsecond electrode 14. The first electrode 12, therefore, is preferablyelectrically grounded by the grounding connection 34 and ground 36.

FIGS. 2, 4, 5, 5A, 6, 7, and 8 illustrate similar pressure modules 10constructed according to the invention, and which show other preferredfeatures of the invention. These figures show the sensing modules 10 inschematic cross-sectional views; yet it is to be understood that suchviews are to be interpreted in the context of a three-dimensionalstructure. For example, the modules 10 of FIGS. 1, 2, 4, 5, 5A, 6, 7 and8 are generally rectangular in shape, each having anothercross-sectional view that is similar in size, and perpendicular to, theassociated illustrated view. Further understanding of athree-dimensional process module may be made with reference to FIG. 2,of U.S. application Ser. No. 08/084,415, entitled "Method and Apparatusfor Inverting Samples in a Process", which is incorporated herein byreference.

Among other factors, high deposition rates result from smaller electrodespacings; yet it is increasingly difficult to ignite plasma atincreasingly smaller electrode spacings, particularly under tenmillimeters. Therefore, the invention provides for the ionization of gasinjected into the gap 18 of this small size, as described below. FIG. 2,for example, illustrates the module 10b connected to a pressure pump 42at the gas outlet 26; and further including linear actuators 44connected to the first electrode 12. Either of the structures 42 and 44are independently operable in conjunction with other elements of themodule 10b to ignite gas within the gap 18; or, alternatively, theycooperate together with the other elements of the module 10 to ignitethe gas.

More particularly, the transformation of gas molecules such as silane,hydrogen, ammonia and nitrogen into a plasma--i.e., the ignition and thecreation of a plasma--and, once ignited, the continual maintenance ofthe plasma within the module 10b are functionally dependent upon modulepressure and electrode spacing. FIG. 3, for example, qualitativelyillustrates plasma maintenance curves for two typical gases, Gas 1 andGas 2, as the relation between the voltage differential required betweenthe electrodes to maintain a discharge and the product of pressure andelectrode spacing. The product of pressure and electrode spacingrepresents the horizontal axis. The voltage differential requiredbetween the electrodes to maintain a discharge within the gap 18represents the vertical axis.

The shape of these two plasma maintenance curves, Gas 1 and Gas 2, canbe understood with reference to the ionization physics within the module10b. To ignite and maintain a plasma within the module 10b, electronsmust be accelerated within the gap 18 to energies in which thecollisions of the accelerated electrons with the gas molecules in thegap 18 cause ionization of the gas molecules, i.e., the creation of moreelectrons and ions. The probability for an electron to cause ionizationis directly proportional to the pressure within the module 10b and thedistance traveled by the electron in the plasma, as controlled by theelectrode spacing. Relative to Gas 1, for example, if the magnitudes ofthe module pressure and/or electrode spacing are too small, there arenot enough electrons created to ignite the plasma, as illustrated inregion A, FIG. 3. If, on the other hand, the magnitudes of the modulepressure and/or electrode spacing are too great, the electrons are notaccelerated to energies sufficient to cause ionization because ofinelastic collisions with gas molecules, as illustrated in region B,FIG. 3. Therefore, there is a condition 43 wherein the product of themodule pressure and of the electrode spacing is such that the voltagerequired to ignite the plasma is at a minimum for the illustrated Gas 1.

According to the invention, the preferred operating conditions fordepositing film from typical Gas 1 occur in region A, FIG. 3. By way ofexample, a plasma of hydrogen and silane is ignited in one instance atpressures above 0.6 Torr; however, below that pressure, no plasma isignited. Nevertheless, after ignition at 0.6 Torr, the pressure isreduced to as low as 0.1 Torr, since the required voltage to maintain adischarge, and hence maintain the plasma, is lower than the voltagerequired to ignite, and thereby initiate, the plasma.

As also illustrated in FIG. 3, the minimum plasma maintenance voltagefor Gas 1, i.e. Vmin, relative to the pressure-electrode product, i.e.(pd)min, is gas dependent. Thus, in accordance with the invention, theelectrode spacing and/or module pressure are modified as a function ofthe particular gas within the module 18 to enable ignition, i.e., thetransformation of those gas molecules into the plasma 30.

These gases are generally classified into two groups: deposition gasesand etch gases. The preferred deposition gases used in accordance withthe invention are silane (SiH₄), disilane (Si₂ H₆), hydrogen (H₂),ammonia (NH₃), phosphine (PH₃), nitrogen (N₂), nitrogen trifluoride(NF₃), helium (He), argon (Ar), carbon tetrafluoride (CF₄),hexafluoroethane (C₂ F₆), oxygen (O₂), nitrous oxide (N₂ O), methane(CH₄), borane (BH₃), diborane (B₂ H₆), and mixtures thereof (thepreferred deposition mixtures are silane and hydrogen; silane, ammoniaand nitrogen; silane and phosphine; silane and methane; silane andborane; and silane and diborane). The preferred etch gases used inaccordance with the invention are chlorine (Cl₂), sulfur hexafluoride(SF₆), hydrogen chloride (HCl), carbon tetrafluoride (CF₄), hydrogenbromide (HBr), carbon tetrachloride (CCl₄), nitrogen trifluoride (NF₃),carbon dichloride difluoride (CCl₂ F₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), oxygen (O₂), boron tribromide (BBr₃),chlorotrifluoride (ClF₃), fluorine (F₂), and mixtures thereof (thepreferred etch mixtures are carbon tetrafluoride and oxygen;hexafluoride and oxygen; and nitrogen trifluoride and oxygen).

With an electrode spacing of less than approximately ten millimeters,the RF generator 16 generally does not provide sufficient voltage totransform gas molecules injected into the gap 18 into plasma, such asillustrated in region A of FIG. 3. In operation, therefore, the pressurepump 42 of FIG. 2 initially operates as a vacuum pump to evacuate themodule 10b to approximately 0.01 Torr, which reduces oxidation and otherimpurities that might form on the heated samples. Selected depositiongas molecules are then injected into the module 10b, and the pressurepump 42 controls the pressure within the module 10b by operation of thethrottle 41 to between approximately 0.1 to 1 Torr, which represents apressure range that permits ionization of the gas molecules by theprimary RF generator 16. The throttle 41 functions as an orifice withinthe gas outlet 26 to increase or decrease the pressure in the moduleselectively: decreasing the orifice of the throttle 41 increases thepressure within the module 10b; while increasing the orifice decreasesthe module pressure. The pressure pump 42 increases the pressure withinthe module 10b selectively, such as in accordance with the plasmamaintenance curve of FIG. 3, as a function of the gas within the moduleto enable the transformation of the gas molecules into plasma. Once theplasma is ignited, the pump 42 evacuates, or de-pressurizes, the module10 to normal pressures, e.g., 0.1 Torr, which are preferred for uniformhigh rate deposition.

Thus, the pressure pump 42 of FIG. 2 is selectively operable to (i)pressurize the module 10b to enable the transformation of gas moleculesinjected into the gap 18 into plasma and (ii) de-pressurize, e.g.,evacuate, the module 10b to enable high rate deposition at preferredconditions. Those skilled in the art can appreciate that the pump 42 isreadily constructed as a vacuum pump or combination pump and compressorto provide the operations (i) and (ii). The operation betweenpressurization and de-pressurization within the module 10b iscontrollable, in accordance with the invention; and preferably short induration, e.g., 1 second, so that gas is ionized and the module 10breturns to normal evacuated pressures without lengthy pressurization ofthe module 10b.

A similar ignition procedure is realized through operation of the linearactuators 44. The actuators 44 are connected to the first electrode 12to move the electrode 12 and adjust the gap separation "d" selectively.Thus, to ignite the gas within the gap 18, the actuators 44 increase theseparation between the electrodes 12 and 14 to a point, for example inaccordance with FIG. 3, where the RF generator ignites and transformsthe molecules into the plasma 30. Once ionization occurs, the RFgenerator 16 has sufficient voltage to maintain the plasma 30 within thegap 18; thus the actuators 44 reposition the electrode 12 to a gapseparation "d" of less than approximately one centimeter to increase thedeposition rate.

Linear actuators 44 preferably reside within the housing 22 so that themodule 10b is evacuated without required mechanical motion through thesealed module housing wall 20. Therefore, the actuators 44 haveelectrical components (not shown), including wires and electricalfeed-through fittings, which extend from the actuators 44 and throughthe housing 20 for communication to outside of the module 10.Alternatively, although not preferred, the actuators 44 physicallyextend through the evacuated module housing 20 through the use ofmechanical seals (not shown).

In accordance with the invention, the pump 42 and the actuators 44preferably operate simultaneously with the RF generator 16 to ionizegas. The required voltage to transform molecules into plasma decreasesmore rapidly by simultaneously increasing module pressure and electrodespacing. Therefore, the undesirable time duration away from idealdeposition conditions, e.g., 0.1 Torr and 1-9 mm gap separation, isdecreased.

The invention thus provides the pump 42 and the actuators 44 to aid inthe ionization of gas injected between the electrodes. The generator 16cannot generally ionize gas at low pressures, e.g., 0.1 Torr, and atsmall gap separations, e.g., 1-9 mm, without the further features of theinvention, e.g., the pump 42, the actuators 44, and other practicesaccording to the invention taught below. The module 10b has a totalfixed capacitance that is dependent, in part, upon the module geometry,e.g., the electrode gap spacing. Thus, for example, at the highfrequency of the first RF generator 16, the total fixed modulecapacitance has a relatively low associated impedance. This translatesto a low voltage differential between the electrodes 12 and 14 for agiven power. For ignition purposes, only voltage is important; hence therequirement for additional features according to the invention, such asthe pump 42 and actuators 44.

FIG. 4 shows a module 10c with the addition of an electron source 60, inaccordance with the invention. The electron source 60 has a hot filament62, operating at a low voltage and at approximately 10 amps, and emits abeam of electrons 64 through an extraction slit 66 and into the gap 18.The slit 66 is approximately ten millimeters in length and onemillimeter in height, and is parallel to the gap 18. The filament 62 isat a negative potential relative to the grounded extraction slit 66 to"extract" the electrons from the filament 62 surface. Accordingly, gasinjected into the gap 18 is ionized at a selective time by applying apositive voltage to the slit 66 once the filament 62 is emittingelectrons.

Preferably, the electrons 64 emitted by the filament 62 have energies ofapproximately 300 eV when the filament is positioned at approximately 70mm or less from the gap 18. If the filament 62 is positioned nearer tothe gap 18, less energy is acceptable. However, if the filament 62 ispositioned further from the gap 18, more energy is required. Thisphenomenon occurs because the emitted electrons 64 lose energy duringtravel to the gap 18, such as through collisions with other molecules inthe module 10c. Additionally, different gases within the module 10crequire different ionization energies. Therefore, the invention providesfor differing electron illumination energies by changing the negativepotential of the filament 62. This potential is adjusted by changing thevoltage between the filament 62 and slit 66 to alter the energy of theemitted electrons.

Those skilled in the art will appreciate that other electron sourcesfall within the scope of the invention. For example, electron sourcesexist which are manufactured without the filament 62 of FIG. 4, andwhich are suitable for igniting the gas within the gap 18. Theprobability of ionizing gas within the gap 18 is at a maximum whenelectrons with energies between approximately 50 eV and 100 eV collidewith the gas molecules in the gap 18. Therefore, other electron sourceswhich are operable to generate electrons with energies betweenapproximately 50 eV as 100 eV within the gap 18 are also suitable forionizing gas therein in accord with the invention.

FIGS. 5 and 5A illustrate one preferred practice of the invention inwhich an ultraviolet (UV) source 70, 72 illuminates and ignites gaswithin the gap 18 with UV photons: FIG. 5 shows the UV source 70, 72within the evacuated module 10d; while FIG. 5A shows the UV source 70,72 outside the module 10e, in standard atmosphere.

More particularly, FIG. 5 illustrates the module 10d with a UV lamp 70that emits UV photons. A reflector 72 surrounds the lamp 70 such thatemitted photons are collected and are emitted only through the aperture74, which directs the UV photon beam 76 generally towards the gap 18.The beam 76 is substantially collected and collimated by UV transmittingoptics 78 such that the beam 76 illuminates the gap 18 withoutsignificant loss of intensity.

Similarly, FIG. 5A shows a UV source 70, reflector 72, aperture 74, beam76, and optics 78, and additionally shows an optical interface 79 thattransmits UV radiation and which preferably withstands operationalpressure differentials. The interface 79 is required because the UVsource 70 of FIG. 5A is positioned outside the process module 10e, e.g.,exterior to the pressure-sealing housing 20'. UV energy cannot transmitthrough materials which typically form the housing 20', e.g., aluminum,and thus a glass-like interface 79 is required.

Preferably, the photon energies emitted by the UV source 70, 72 of FIGS.5 and 5A are greater than approximately five electron volts.

FIGS. 6 shows another preferred practice of the invention in which asparking source 80 generates a spark to ignite the gas within the gap18. The sparking source has a power supply 82 connected to a spark gap84, and is preferably electrically isolated from the electrodes 12 and14. In operation, the power supply 82 applies a voltage across the gap84 selectively to generate a spark. The sparking source is arrangedwithin the module 10f such that generated sparks are introduced into thegap 18, thus ionizing gas therein. Preferably, therefore, the gap 84 hasa line of sight 86 directly to the gap 18. A spark is generally inducedby a voltage differential across the gap 84 in the kilovolt range andthus generates electrons with high energies. These electrons emit in alldirections and many collide with the electrodes 12 and 14, which in turnrelease secondary electrons that ignite the gas within the gap 18. Thesesecondary electrons include electrons with maximum ionizationprobabilities for a given gas, e.g., those electrons with energiesbetween approximately 50 eV and 100 eV.

FIG. 7 shows another preferred practice of the invention for ignitinggas within a process module 10g. In FIG. 7, the process module 10g has asecond RF generator 90 connected to the second electrode 14 via RF line92. The second RF generator 90 has a second frequency, preferably 400kHz, which is less than the preferred primary frequency of 60 MHz. Thesecond RF generator 90 is selectively operable to apply RF energy at thelower frequency to the gap 18. The second RF generator 90 is energizedonly for a short duration, at a time sufficient to ionize gas within thegap 18. Alternatively, the generator 90 is repetitively pulsed atapproximately once per second to ionize the gas.

In operation, the primary 60 MHz energy combines with the lower 400 kHzenergy to ignite and ionize the gas. Each generator 16 and 90 providesapproximately equal energy, e.g., 0.05 W/cm² to 1 W/cm² within the gap18; yet, as stated earlier, the high frequency of the first RF generator16 lacks sufficient voltage to ionize gas within a gap 18 spaced at lessthan approximately one centimeter. The lower frequency generator 90, onthe other hand, provides relatively high voltage across the electrodes.At lower RF frequencies, such as at 400 kHz, the module capacitance hasa much higher associated impedance and greater peak-to-peak voltage ascompared to a 60 MHz frequency. A high voltage generates high electronenergies with a corresponding increase in ionization probability.Accordingly, the generators 90 and 16 preferably combine to generate apeak-to-peak voltage between approximately one kilovolt and fivekilovolts in the gap 18.

One advantage of the second RF generator 90 is that, unlike otherignition techniques described herein, no internal modifications to themodule 10 are needed to ignite the gas. The generator 90 is external tothe housing 20 and connects with the second electrode 14 through the RFline 92. In one practice of the invention, the line 92 is coaxial withthe RF line 28, to reduce the required cabling, which pierces theevacuated housing 20 at a pressure-tight RF feed-through fitting.

In one other practice of the invention, the generator 90 is not an RFgenerator; but is instead a high DC power source. As stated above,voltage is an important ionization parameter. Thus, DC source 90, inthis alternative embodiment, generates a spark in the gap selectively toionize gas therein. In operation, the DC source 90 combines with the 60MHz energy to ignite and ionize the gas. After the gas is ionized andmolecules have been transformed into plasma, the DC power source 90 isturned off.

FIG. 8 shows a process module 10h constructed according to a furtherpractice of the invention and which includes an x-ray source 100 andshielding 102. The x-ray source 100 and shielding 102 are constructed bymethods known in the art to generate a beam of x-ray radiation 104 thatextends through at least part of the gap 18. As illustrated, the x-raysource 100 is preferably embedded within one of the electrodes 12 and14', here shown imbedded in electrode 14'. The shielding 102 confinesthe x-ray radiation to the desired beam 104, and thereby protects usersof the process module 10h from the unwanted x-rays.

Alternative to an x-ray source, the source 100 can be a radioactivesource in accordance with the invention. As above, a shielding 102protects users of the module 10 and guides the emitted radioactiveparticles along a path that includes the gap 18.

X-ray and other radioactive radiation provide the energy required toionize gases within the gap 18. Once ionized, the source 100 ispreferably covered, via an automatic mechanical shutter (not shown),since the RF generator 16 has sufficient power to maintain plasma oncethe gas is ionized.

Those skilled in the art can appreciate that the source 100, x-ray orradioactive, may be positioned elsewhere to ionize gas within the gap18. However, it is a convenient packaging to locate the source 100within one of the electrodes 12 and 14', since both x-rays andradioactive energy propagate through the electrode material.

It is to be understood that the apparatus described in connection withFIGS. 1-8 are illustrative rather than limiting, and that additions andmodifications will be apparent to those skilled in the art withoutdeparting from the scope of the claims which follow. Accordingly,variations for ionizing gas within the gap of a PECVD process moduleexist in accordance with the invention, other than those specificallydescribed in connection with FIGS. 1-8.

For example, with reference to FIG. 1, the inlet 24 is used in onepractice of the invention to inject combinations of gases, rather than asingle gas for ionization. Alternatively, a second inlet 24 is installedinto the housing wall 20 to inject additional gases. These additionalgases can act as the catalyst to ignite the primary gas from which theplasma 30 is derived. More particularly, by injecting a second gascomprising helium, xenon, krypton, nitrogen, argon, neon, or otheratomic elements with a primary gas, e.g., silane, electrons liberated byionization of the molecules of the second gas cause further ionizationof the primary gas molecules, e.g., silane, and enable thetransformation of molecules into a plasma. Accordingly, these secondgases are injected for a short duration through the inlet 24 until thegas combination ignites, whereinafter the second gas is inhibited fromflowing through the module so that a deposition from a "pure", e.g.,silane-based plasma results. Preferably, the injected atomic elementshave ionization energies which are less than approximately 25 eV.

Additionally, those skilled in the art can appreciate that the inventionas described herein is also applicable for etching surfaces within aprocess module, for example to "clean" the module. Etch gases such aschlorine (Cl₂), sulfur hexafluoride (SF₆), hydrogen chloride (HCl),nitrogen trifluoride (NF₃), carbon tetrachloride (CCl₄), hydrogenbromide (HBr), carbon tetrafluoride (CF₄), carbon dichloride difluoride(CCl₂ F₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄),boron tribromide (BBr₃), chlorotrifluoride (ClF₃), and fluorine (F₂) aretypically mixed with oxygen (O₂) to form the preferred gaseous mixturethat is ignited according to the further features of the invention. Foretch purposes, the electrode spacing is generally spaced to betweenapproximately one and seventy five millimeters.

Appended to this specification as Appendix A are detailed drawings andassembly documents that present further details of preferredconstructions of process modules that (i) ignite gas and (ii) exposesamples to plasma, in accordance with the invention.

In view of the foregoing, what is claimed as new and secured by theLetters Patent is:
 1. In a sample-processing method for exposing asample to reactive plasma within a process module having first andsecond electrodes, the improvement comprising the steps of(A) arrangingsaid first and second electrodes within the module to form asubstantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, and (D) applying radiofrequency energy to said gap,said radiofrequency energy having a primary frequency which transformsmolecules into plasma, wherein the step of ionizing said gas comprisesthe steps of generating ultraviolet radiation with an ultraviolet sourceand directing said ultraviolet radiation between said electrodes.
 2. Amethod according to claim 1, the further improvement comprising thesteps of providing an UV interface window that is transmissive to saidultraviolet radiation and directing said ultraviolet radiation throughsaid UV interface window to said gap.
 3. In a sample-processing methodfor exposing a sample to reactive plasma within a process module havingfirst and second electrodes, the improvement comprising the steps of(A)arranging said first and second electrodes within the module to form asubstantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, and (D) applying radiofrequency energy to said gap,said radiofrequency energy having a primary frequency which transformsmolecules into plasma, wherein the step of ionizing said gas comprisesthe step of applying a second radiofrequency energy to said gap, saidsecond radiofrequency energy having a second frequency that is less thansaid primary frequency.
 4. A method according to claim 3, the furtherimprovement wherein said second frequency is approximately 400 kHz.
 5. Amethod according to claim 3, the further improvement comprising the stepof pulsing said second radiofrequency energy selectively.
 6. A methodaccording to claim 3, the further improvement comprising the step ofinhibiting said second radiofrequency energy selectively after gas isionized within said gap.
 7. In a sample-processing method for exposing asample to reactive plasma within a process module having first andsecond electrodes, the improvement comprising the steps of(A) arrangingsaid first and second electrodes within the module to form asubstantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, and (D) applying radiofrequency energy to said gap,said radiofrequency energy having a primary frequency which transformsmolecules into plasma, wherein the step of ionizing said gas comprisesthe steps of connecting a direct current source to said second electrodeand generating a physical spark in said gap.
 8. In a sample-processingmethod for exposing a sample to reactive plasma within a process modulehaving first and second electrodes, the improvement comprising the stepsof(A) arranging said first and second electrodes within the module toform a substantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, (D) applying radiofrequency energy to said gap, saidradiofrequency energy having a primary frequency which transformsmolecules into plasma,wherein the step of ionizing said gas includes thestep of pressurizing said module selectively such that said moleculesare transformed into plasma by said first radiofrequency energy, and (E)de-pressurizing said module after said molecules are transformed intoplasma by said first radiofrequency energy.
 9. In a sample-processingmethod for exposing a sample to reactive plasma within a process modulehaving first and second electrodes, the improvement comprising the stepsof(A) arranging said first and second electrodes within the module toform a substantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, and (D) applying radiofrequency energy to said gap,said radiofrequency energy having a primary frequency which transformsmolecules into plasma, wherein the step of ionizing said gas comprisesthe steps of generating a beam of x-ray radiation and directing saidradiation into said gap.
 10. A method according to claim 9, the furtherimprovement comprising the step of shielding said x-ray radiation suchthat said x-ray radiation is substantially confined to one or more pathsextending through said gap.
 11. In a sample-processing method forexposing a sample to reactive plasma within a process module havingfirst and second electrodes, the improvement comprising the steps of(A)arranging said first and second electrodes within the module to form asubstantially uniform gap separating said electrodes of betweenapproximately one and ten millimeters, (B) injecting gas into said gap,said gas being selected from the group consisting of silane (SiH₄),disilane (Si₂ H₆), hydrogen (H₂), ammonia (NH₃), phosphine (PH₃),nitrogen (N₂), nitrogen trifluoride (NF₃), helium (He), argon (Ar),carbon tetrafluoride (CF₄), hexafluoroethane (C₂ F₆), oxygen (O₂),nitrous oxide (N₂ O), methane (CH₄), borane (BH₃), diborane (B₂ H₆), andmixtures thereof, and being suitable for ionization and fortransformation into plasma when exposed to radiofrequency energy, (C)ionizing said gas, and (D) applying radiofrequency energy to said gap,said radiofrequency energy having a primary frequency which transformsmolecules into plasma, wherein the step of ionizing said gas comprisesthe step of generating ionizing particles in said gap from a radioactivesource.
 12. A method according to claim 11, the further improvementcomprising the step of shielding said radioactive source such that saidionizing particles are substantially confined to one or more pathsextending through said gap.